Method of operating a communication system

ABSTRACT

A method of operating a communication system is provided. The communication system comprises a NetWork (NW) and User Equipment (UE), a Non Access Stratum (NAS) via which the NW and UE communicate with each other, and an Access Stratum (AS) providing a physical connection to enable the NAS communication between the NW and UE to be achieved, the physical connection including a wireless radio link to the UE. The method includes sending information from the NW to the UE to determine at least one action of the UE in response to a failure of a radio link to the UE, detecting a Radio Link Failure (RLF), in response to the detecting of the RLF, deciding on an action to be performed by the UE according to the received information, and performing, by the UE, the decided action.

PRIORITY

This application claims the benefit under 35 U.S.C. §119(a) of a patentapplication filed on Oct. 6, 2008 in the United Kingdom IntellectualProperty Office and assigned Serial No. 0818243.8, the entire disclosureof which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a telecommunications system. Moreparticularly, the present invention relates to a method and apparatusfor recovering a radio link after Radio Link Failure (RLF) in atelecommunications system.

2. Background to the Invention:

Communication systems (which may also be described as telecommunicationssystems) that provide wireless connections to mobile User Equipment (UE)are well known and have a variety of configurations. Research anddevelopment of communication systems is ongoing, especially in the fieldof technical specifications for so-called Third Generation PartnershipProject (3GPP) systems.

In the 3GPP systems, communication between a UE and a Core Network (CNor NW) is in a so-called Non-Access Stratum (NAS) (in other words,communication is at the NAS level) wherein an Access Stratum (AS)provides services for the NAS. One such service that the AS provides,and seeks to maintain, is a physical connection to enable the NAS layercommunication between the CN and the UE. This physical connectionincludes a radio link to the UE (i.e., a wireless link). However, anissue arises in the event of a Radio Link Failure (RLF). Moreparticularly, an issue arises regarding how the system should respond toan RLF, and what steps, if any, should be taken in order to re-establisha connection.

The Radio Resource Control (RRC) re-establishment procedure after RLF orhandover failure is not determined yet in 3GPP Long Term Evolution(LTE). One possibility to address RLF or handover failure, afterdetection at the lower layer, is for the UE to take the followingactions:

-   -   1. The UE starts a timer T311 and performs cell selection.    -   2. If the UE finds a suitable Evolved Universal Terrestrial        Radio Access (E-UTRAN) cell (i.e., an accessible cell having        sufficient radio quality), the UE initiates a re-establishment        procedure by sending the RRC Connection Re-establishment Request        message.    -   3. If an enhanced Node B (eNB) cell receiving the RRC Connection        Re-establishment Request is “prepared” (i.e., either was the        source cell or a cell that was prepared by the source cell with        the UE context through the handover preparation procedure), the        re-establishment procedure succeeds.    -   4. If the eNB cell receiving the RRC Connection Re-establishment        Request is not “prepared”, the UE is rejected and pushed back to        RRC_IDLE. Further connection recovery is left to NAS.    -   5. If the UE cannot find a suitable E-UTRAN cell, the UE can        search for UTRAN/GSM/EDGE Radio Access Network (GERAN) cells. If        the UE finds a suitable UTRAN/GERAN cell, the UE returns to        RRC_IDLE. Further connection recovery is left to (UTRAN/GERAN)        NAS.    -   6. If the UE cannot find a suitable cell before T311 expiry, the        UE returns to RRC_IDLE mode.

In the event that the UE reaches steps 4, 5 or 6, it is expected thatthe connection would be recovered by the NAS (e.g., by NAS initiating anew NAS Service Request procedure). However, this is only likely if theUE had pending Up Link (UL) data. In the event that there was no pendingUL data, but there was Down Link (DL) data in the network side, the NASrecovery procedure will not be triggered. Hence, one possible optionwould be to indicate the specific case of radio failure to NAS, so thatNAS can initiate the recovery procedure in both cases (i.e. whether ornot there was UL data remaining to be sent). Note that after step 5, itis also expected that the NAS will recover the connection by aninter-RAT recovery procedure.

A possibility, involving NAS specific recovery, is that the NAS can,after recovery from RLF (made known to the NAS by the AS) trigger aService Request or a Tracking Area Update (TAU)_Request to ensure thatthe connection to the network is reestablished.

The main driver for this technique would be that the network might haveDL data waiting to send to the UE. At present (that is, according to thecurrent state of 3GPP discussions) the NAS, after RLF, will remain inIDLE (i.e. an idle mode, or idle state) if there is no UL data to send.The UE is in IDLE mode when no NAS signalling connection between the UEand the network exists. Only if the UE has UL data to send will the NASautonomously perform or trigger a request for radio bearers afterrecovery from RLF. So, in the event that there is DL data, that DL datawill only reach the UE after a paging procedure. That may beunacceptable to certain operators because it involves too long a delayor due to other problems.

In addition to the above scenario, another possible scenario is asfollows:

-   -   1. Cell A and cell B are located within the same Tracking Area        (TA) and both cells are connected to same eNB and Mobility        Management Entity (MME).    -   2. UE is in cell A, in a media session with the network and data        exchange is ongoing.    -   3. There is a radio level failure and RRC attempts to recover.    -   4. RRC, in an attempt to recover, performs cell reselection onto        cell B.    -   5. During this period, there is no UL data pending. There is no        TAU, as cell A and cell B are within the same TA.    -   6. There is a network equivalent of T311 and that network timer        is still running. Until the network equivalent of T311 expires,        the MME is not aware that the UE is no longer in cell A.    -   7. DL data arrives at the network. The network will pass the DL        data to the eNB to deliver through cell A.

In this scenario, even though the UE has recovered itself, it is in anew radio area but the network mistakenly recognizes the UE as being inthe old (previous) radio area. In this scenario, the paging from the NWto deliver DL data will fail and the paging would be escalated toinclude a wider radio area.

From the above examples and discussion of the identified problems, itwill be appreciated that some of these techniques post-RLF may result inan unacceptable service for the user. Hence, an NAS solution (done bytriggering Service Request, TAU or Routing Area Update (RAU)) (in otherwords a NAS specific recovery) after detection of an RLF may provide themeans to overcome loss of this possible anticipated DL data, and henceprovide improved user services.

On the other hand, such automatic use of NAS specific recovery also hasdisadvantages associated with it, as will be appreciated from thefollowing:

-   -   1. The network equivalent of T311 (for sake of description        called T311-nw) can be properly configured to allow the NW to        know if the UE is no longer in the previous radio area (i.e. the        51 connection can be taken down upon expiry of T311-nw). So,        subsequent paging will immediately include a wider area.    -   2. The paging and UE responding will only take approximately 2        or 3 seconds and services will not be severely affected. For        Guaranteed Bit Rate (GBR) services, it is likely that UL data        will be transmitted very soon and that in itself will cause the        UE to trigger a Service Request. On the other hand, non-GBR        services can, by their nature, suffer seconds of interruption.    -   3. The complexities may not outweigh the gain. The gain may be a        matter of 6 to 8 seconds compared to 13 seconds if nothing is        done. It could be argued that if T311-nw is set correctly, to 5        seconds, for example, then with 2-3 seconds of paging delay that        will only take 7 to 8 seconds, and that is comparable to doing        all the complexities to get to 6 to 8 seconds.    -   4. The performance of NAS signalling, be it a Service Request, a        TAU request or an RAU request, is in anticipation of there being        DL data to be transmitted to the UE. There is no certainty,        however, that there will be DL data.    -   5. Performing the NAS signalling after detection of a RLF will        cause a large signalling spike in certain conditions, for        example in the situation where a trainload of users go into and        then come out of a tunnel.

Thus, automatic performance of NAS restoration procedures after a RLFcan have disadvantages associated with it and adoption of such aprocedure may not suit all operators and all situations. Accordingly,there is a need for an improved method for recovering from an RLF.

SUMMARY OF THE INVENTION

An aspect of the present invention is to address at least theabove-mentioned problems and/or disadvantages and to provide at leastthe advantages described below. Accordingly, an aspect of the presentinvention is to provide a method for improved recovery from a Radio LinkFailure (RLF).

According to an aspect of the present invention, a method of operating acommunication system comprising a NetWork (NW) and User Equipment (UE),a Non Access Stratum (NAS) via which the NW and the UE communicate witheach other, and an Access Stratum (AS) providing physical connections toenable the NAS communication between the NW and the UE to be achieved,the physical connections including a wireless radio link to the UE, isprovided. The method includes sending information from the NW to the UEto determine at least one action of the UE in response to a failure of aradio link to the UE, detecting a Radio Link Failure (RLF), in responseto the detecting of the RLF, deciding on an action to be performed bythe UE according to the information, and performing, by the UE, thedecided action.

According to another aspect of the present invention, a method ofoperating a communication system comprising a core NetWork (NW), UserEquipment (UE), and an Access Network (AN) providing a physicalconnection to enable the communication between the NW and the UE to beachieved, the physical connection including a wireless radio link to theUE, is provided. The method includes sending information from the NW tothe UE to determine at least one action of the UE in response to afailure of a radio link to the UE detecting an RLF, in response to thedetecting of the RLF, deciding on an action to be performed by the UEaccording to the information, and performing, by the UE, the decidedaction.

According to yet another aspect of the present invention, a method ofoperating a communication system comprising a NetWork (NW) and a UserEquipment (UE), a Non Access Stratum (NAS) via which the NW and the UEcommunicate with each other, and an Access Stratum (AS) providing aphysical connection to enable the NAS communication between the NW andthe UE to be achieved, the physical connection including a wirelessradio link to the UE, is provided. The method includes detecting a RadioLink Failure (RLF) by the AS, informing the NAS of the RLF, determiningif a message is received by the UE regarding recovery by the NAS fromthe RLF, if it is determined that the message regarding recovery by theNAS from the RLF is received, determining if the received messageindicates that recovery by the NAS from the RLF is permitted, and, if itis determined that the recovery by the NAS from the RLF is permitted,performing a recovery from the RLF by the NAS.

It will be appreciated that, in its broadest sense, the inventive methodis not limited to any particular action or inaction by the UE.Furthermore, exemplary embodiments of the invention may utilize theconcept of remote setting or configuration of the UE (in terms of whataction the UE performs in response to a RLF) by the NW.

Other aspects, advantages, and salient features of the invention willbecome apparent to those skilled in the art from the following detaileddescription, which, taken in conjunction with the annexed drawings,discloses exemplary embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features, and advantages of certainexemplary embodiments of the present invention will be more apparentfrom the following description taken in conjunction with theaccompanying drawings, in which:

FIG. 1 is a schematic representation of a communication system accordingto an exemplary embodiment of the present invention;

FIG. 2 is a flowchart illustrating a conventional method of operating acommunication system;

FIG. 3 is a flowchart illustrating a possible alternative method ofoperating a communication system;

FIG. 4 is a flowchart illustrating a method of operating a communicationsystem according to an exemplary embodiment of the present invention;

FIGS. 5 to 9 illustrate methods for sending messages according toexemplary embodiments of the present invention; and

FIGS. 10 and 11 illustrate the System Architecture Evolution (SAE)/LongTerm Evolution (LTE) reference architecture, as defined by 3GPP, and inwhich methods embodying the invention may be implemented.

Throughout the drawings, it should be noted that like reference numbersare used to depict the same or similar elements, features, andstructures.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The following description with reference to the accompanying drawings isprovided to assist in a comprehensive understanding of exemplaryembodiments of the invention as defined by the claims and theirequivalents. It includes various specific details to assist in thatunderstanding but these are to be regarded as merely exemplary.Accordingly, those of ordinary skill in the art will recognize thatvarious changes and modifications of the embodiments described hereincan be made without departing from the scope and spirit of theinvention. In addition, descriptions of well-known functions andconstructions are omitted for clarity and conciseness.

The terms and words used in the following description and claims are notlimited to the bibliographical meanings, but, are merely used by theinventor to enable a clear and consistent understanding of theinvention. Accordingly, it should be apparent to those skilled in theart that the following description of exemplary embodiments of thepresent invention are provided for illustration purpose only and not forthe purpose of limiting the invention as defined by the appended claimsand their equivalents.

It is to be understood that the singular forms “a,” “an,” and “the”include plural referents unless the context clearly dictates otherwise.Thus, for example, reference to “a component surface” includes referenceto one or more of such surfaces.

By the term “substantially” it is meant that the recited characteristic,parameter, or value need not be achieved exactly, but that deviations orvariations, including for example, tolerances, measurement error,measurement accuracy limitations and other factors known to those ofskill in the art, may occur in amounts that do not preclude the effectthe characteristic was intended to provide.

FIG. 1 is a schematic representation of a communication system accordingto an exemplary embodiment of the present invention.

Referring to FIG. 1, a communication system comprises a Core Network(CN) 102 (which, in certain exemplary embodiments, may be described as aNetWork NW), a User Equipment (UE) 104, and an Evolved UniversalTerrestrial Radio Access Network (E-UTRAN) system 106 between the CN 102and the UE 104. Communication between the UE 104 and the CN 102 is in aNon-Access Stratum (NAS) 108. An Access Stratum (AS) 110 providesservices for the NAS 108. That is, the AS 110 provides a physicalconnection 114 (i.e., radio interface) to enable the NAS communicationbetween the NW 102 and the UE 104. The physical connection 114 includesa wireless radio link to the UE 104 (in other words, the AS 110 providesa physical radio interface between the UE 104 and E-UTRAN 106 in thisexample). The AS 110 also provides a physical link for a CN/E-UTRANinterface 112.

In an exemplary implementation, the system shown in FIG. 1 is arrangedto perform a method comprising:

-   -   1. sending information from the NW to the UE to determine at        least one action of the UE in response to the failure of a radio        link to the UE;    -   2. detecting a Radio Link Failure (RLF);    -   3. in response to detecting the RLF, deciding on an action to be        performed by the UE according to the received information; and    -   4. performing, with the UE, the decided action.

The decided action may, for example, include one or more NAS signallingrecovery actions or procedures. Whether NAS signalling recovery isperformed, or at least triggered, may thus be dependent upon whatinformation the UE has previously received from the network. The networkmay have sent information (e.g., a signal) to inhibit/prevent NASsignalling recovery after an RLF, or alternatively to enable suchaction. Thus, certain exemplary embodiments provide methods to limit theNAS signalling recovery after an RLF for operators who either do notwant it or see it as a disadvantage. These methods provide the networkwith means to remotely set or configure the UE, in terms of what actionit takes after detection of a RLF by the AS. In other words, certainexemplary embodiments provide a way to enable or disable this NASsignalling recovery after detection of RLF.

FIG. 2 is a flowchart illustrating a conventional method of operating acommunication system.

Referring to FIG. 2, the UE is in a media session, including thesending/receiving of UL/DL data, with a network in step 202. The AS isin Radio Resource Control (RRC)_connected mode, and the NAS is inEvolved Mobility Management (EMM)_connected mode. The UE is in theRRC_connected mode when an RRC connection has been established. The UEis in the EMM_connected mode when an NAS signalling connection betweenthe UE and the network is established. In step 204, the AS detects RLFand takes action to recover. In step 206, the AS recovery action endsaccording to modes of the UE and the NAS.

If it is determined that the UE has UL data to send in step 208, the NASsends a SERVICE_REQUEST message to request radio bearers for UL data instep 210. In step 212, the NAS moves to EMM_connected mode. The UE is inthe EMM_connected mode when an NAS signalling connection between the UEand the network is established. The term EMM_connected mode used in thepresent description may correspond to the term ECM_connected state usedin 3GPP TS 23.401. If it is determined in step 208 that the UE does nothave UL data to send, the NAS stays in EMM_IDLE to await a networkaction in step 214 and then returns to step 208. If the network has DLdata, the network pages the UE in step 216 and the NAS moves toEMM_connected mode in step 218.

FIG. 3 is a flowchart illustrating a conventional method of operating acommunication system.

Referring to FIG. 3, NAS specific recovery is automatically performedafter an RLF. In this figure only the triggering of the SERVICE_REQUESTmessage is illustrated for ease of description. More specifically, FIG.3 illustrates a technique for an NAS specific recovery by NASsignalling. That is, the SERVICE_REQUEST message is unconditionallytriggered by the UE. A disadvantage of this method is the unwarrantedand undesirable signalling load. Moreover, this way of NAS specificrecovery is done in anticipation that there will be DL data, which mightnot be the case at all. Accordingly, allocated radio resources will bewasted.

In step 302, the UE is in a media session with a network including thesending/receiving of UL/DL data. The AS is in RRC_connected mode, andthe NAS is in EMM_connected mode. The UE is in the RRC_connected mode,when an RRC connection has been established. The UE is in theEMM_connected mode when an NAS signalling connection between the UE andthe network is established. In step 304, the AS detects RLF and takesaction to recover. In step 306, the AS recovery action ends according tomodes of the UE and the NAS.

In step 308, the AS indicates to the NAS that there has been an RLF. Instep 310, on being informed of the RLF, the NAS sends a SERVICE_REQUESTmessage to request radio bearers regardless if needed. In step 312, theNAS moves to EMM_connected mode.

In contrast to the conventional methods shown in FIGS. 2 and 3,exemplary embodiments of the invention are able to allow networks andoperators to have a choice as to whether there will be an NAS specificrecovery. In giving the network and the operator the choice, the networkoperator can choose to accept the potential disadvantages and thus risksof NAS specific recovery.

FIG. 4 is a flowchart illustrating a method of operating a communicationsystem according to an exemplary embodiment of the present invention.

Referring to FIG. 4, steps 402, 404, 406 and 408 are substantially thesame as steps 302, 304, 306 and 308, respectively, of FIG. 3 andtherefore will not be described here for sake of convenience. In step410, the UE determines if the network provides an indication to the UEregarding whether the UE can perform NAS specific recovery for recoveryfrom an RLF. For example, the UE determines whether it has received anNAS_specific_signalling_for_recovery message indicating that NASspecific recovery is available.

It is to be understood that the indication “NASspecific_signalling_for_recovery” is merely for example and may betermed something else.

If it is determined in step 410 that an indication message has beenprovided, in step 412, the UE determines if the “NASspecific_signalling_for_recovery” message indicates that the UE canperform NAS specific recovery after recovery from an RLF.

If it is determined in step 412 that the “NASspecific_signalling_for_recovery” message indicates that NAS specificrecovery is allowed, then the UE proceeds to step 414 and subsequentsteps in which the UE can trigger NAS specific recovery after recoveryfrom RLF. Steps 414 and 416 are substantially the same as steps 310 and312, respectively, of FIG. 3 and will not be described here for sake ofconvenience.

On the other hand, if it is determined in step 410 that the network doesnot provide an “NAS specific_signalling_for_recovery” message indicatingthat NAS specific recovery is allowed or, if it is determined in step412 that the “NAS specific_signalling_for_recovery” message from thenetwork indicates that the UE is not allowed to perform NAS specificrecovery after recovery from RLF, then the NAS will not perform NASspecific recovery after recovery from the RLF. In either of thesesituations, the UE executes a procedure substantially the same as theprocess of step 208 and subsequent steps of FIG. 2.

FIG. 4 thus shows an exemplary embodiment in which an indication toallow or disallow NAS specific recovery is provided to the UE by the NW.

FIGS. 5 to 9 illustrate methods for sending messages according toexemplary embodiments of the present invention.

In exemplary embodiments of the present invention, the NW can providethe information (indication) to the UE in a variety of ways. Exemplarymethods of providing this “NAS_specific_signalling_for_recovery”indication are shown in FIGS. 5 to 8.

Referring to FIGS. 5-8, various methods of providing the information fordecision-making purposes (e.g. the indication of need for NAS signallingrecovery) are illustrated. For example, any of an Attach Accept message,a TAU Accept message, an RAU Accept message, a GUTI Reallocation messageand other appropriate NAS signalling may provide the “NASspecific_signalling_for_recovery”. These messages will indicate to theUE whether the NAS signalling to recover connection to NW after RLF isto be done. In other words, the information can be included in/sent withany of an Attach Accept message 502 from an MME, a TAU Accept message602 from an MME, an RAU Accept message 702 from a Serving GPRS SupportNodes (SGSN), a Globally Unique Temporary Identity (GUTI) Reallocationmessage 802 from an SGSN and the like.

Certain exemplary embodiments include the providing of the indication atthe AS level.

In exemplary methods below, this indication given at the AS level isconveyed to the NAS through implementation of specific means and willnot be described in detail.

In certain exemplary embodiments the indication is provided over abroadcast channel or by AS signalling.

Referring to FIG. 9, RRC signalling provides the “NASspecific_signalling_for_recovery” indication to the AS of the UE oncereceived from an eNB in response to RRC signalling from the UE to theeNB. After receipt, the AS of the UE will pass this “NASspecific_signalling_for_recovery” indication up to the NAS to indicatewhether NAS signalling to recover the connection is required afterdetecting an RLF. The internal AS to NAS communications are subject toimplementation dependencies and are not discussed in further detailhere.

In certain exemplary embodiments, the information (e.g., the indication)is provided to the UE through an Over The Air (OTA) mechanism. By an OTAmechanism, an indication can be provided to the UE. Such an OTAmechanism, which may be used in an exemplary embodiment of the presentinvention, may include, but is not limited to, the following:

1. Short Messaging Service (SMS)

2. Open Mobile Alliance Device Management (OMA DM)

It will be appreciated from the above that certain NAS restorationprocedures following an RLF indication from the AS may not involve theUE first returning to idle mode. That is, in certain exemplaryembodiments, the UE may return to idle, but in alternative exemplaryembodiments it does not. A possible reason for not returning to idlemode is that there are circumstances (e.g., Guaranteed Bit Rate (GBR) DLstreamed video via User Datagram Protocol (UDP)) in which the UE willhave no data to send in the UL direction, and so will not immediatelysend a SERVICE_REQUEST message to re-establish the dedicated EnhancedPacket Core (EPC) Bearer following the RLF. In that case it could beargued that, as the UE would return to idle mode, it would be necessaryfor the network to page the UE (potentially via a cell in which the UEis no longer present) and then for the UE to re-establish the RRCconnection before the downlink data flow can resume. This could lead tounacceptable data interruption times for GBR services and unsatisfactoryuser experience. Furthermore, paging might not reach the UE if the UEhas moved to another cell and the NW is not (yet) informed. So pagingescalation is needed, adding a further few seconds of delay.

A concern is that if the UE automatically invokes NAS restorationprocedures on receiving the RLF indication from the AS, this could leadto a spike in signalling to the network if a large group of users in thesame cell simultaneously lose coverage and then all attempt restorationprocedures at the same time. One example is that of a train load ofusers entering and leaving a tunnel. It is very difficult for the NAS todetermine what kind of service was actually in use at the time of theRLF. The Radio Access Bearer Manager (RABM) may be aware of the type ofservice but the RABM does not have an indication of the RLF. So theentity that does know of the RLF does not know of the service used,while the entity that does know of the service in use does not know thatan RLF has occurred.

Certain exemplary embodiments of the invention give the operators (thenetwork, or core network) a chance to control whether the UE shouldautomatically attempt NAS restoration procedures on detection of an RLFfrom the AS, and provide that this indication may be communicated to theUE via a system information broadcast, explicit NAS signalling, OTAprovisioning and the like.

It will be appreciated that certain exemplary embodiments of theinvention are 3GPP specific. However, exemplary embodiments may alsorelate to other wireless systems that effectively comprise both anAccess Network and a Core Network. The Non-Access Stratum is essentiallya generic term to describe the stratum that encapsulates the layer 3(and in some cases layer 4) protocols used for communication between theUE and the Core Network. Likewise, the Access Stratum is a generic termused to describe the stratum that encapsulates the Layer 2/Layer 3protocols used for communication between the UE and the Access Networkand between the Access Network and the Core Network.

The 3GPP specification TS 23.101 defines the Access Stratum as “ . . .the functional groupings consisting of the parts in the infrastructureand in the user equipment and the protocols between these parts beingspecific to the access technique (i.e. the way the specific physicalmedia between the User Equipment and the Infrastructure is used to carryinformation). The Access Stratum provides services related to thetransmission of data over the radio interface and the management of theradio interface to the other parts of Universal MobileTelecommunications System (UMTS). The Access Stratum includes thefollowing protocols:

-   -   1. Mobile Termination—Access Network. This protocol supports        transfer of detailed radio-related information to co-ordinate        the use of radio resources between the MT and the access        network.    -   2. Access Network—Serving Network. This protocol supports the        access from the serving network to the resources provided by the        access network. It is independent of the specific radio        structure of the access network.”

The Non-Access Stratum is defined implicitly in TS 23.110 (UMTS accessstratum services and functions) where it is used to describe theboundaries of the Access Stratum.

Other exemplary methods embodying the invention may be implemented insystems comprising a UE, a Core Network (CN) and an Access Network (AN).In such exemplary embodiments, the CN sends the information to the UE todetermine the course of action on detection of RLF and the RLF isdetected by the UE based on communication (or lack thereof) between itand the AN.

As further background to the invention, FIGS. 10 and 11 illustrateSystem Architecture (SAE)/Long Term Evolution (LTE) Reference NetworkArchitecture.

FIGS. 10 and 11 below—extracted from TS 23.401—provide an illustrationof the SAE/LTE reference architecture as defined by 3GPP.

Referring to FIGS. 10 and 11, the 2G GSM/GPRS system (GERAN+SGSN) andthe 3G UMTS system (UTRAN+SGSN) are illustrated. Through thisillustration, one is able to see the “link-up” from/to the 3GPP 2G and3G system and 3GPP's SAE/LTE.

The ideas, methods and claims described herein refer to EPC and anEnhanced Packet System (EPS) and more particularly refer to possibleInterworking Functions within the EPC. To help visualize the scope ofEPC, FIG. 11 is provided. Regarding FIG. 11, it should be noted that EPCalso encompasses the HSS.

It will be appreciated that exemplary embodiments of the presentinvention can be realized in the form of hardware, software or acombination of hardware and software. Any such software may be stored inthe form of volatile or non-volatile storage such as, for example, astorage device like a ROM, whether erasable or rewritable or not, or inthe form of memory such as, for example, RAM, memory chips, device orintegrated circuits or on an optically or magnetically readable mediumsuch as, for example, a CD, DVD, magnetic disk or magnetic tape or thelike. It will be appreciated that the storage devices and storage mediaare exemplary embodiments of machine-readable storage that are suitablefor storing a program or programs comprising instructions that, whenexecuted, implement embodiments of the present invention. Accordingly,exemplary embodiments provide a program comprising code for implementinga system or method as claimed in any one of the claims of thisspecification and a machine-readable storage storing such a program.Still further, such programs may be conveyed electronically via anymedium such as a communication signal carried over a wired or wirelessconnection and exemplary embodiments suitably encompass the same.

Throughout the description and claims of this specification, the words“comprise” and “contain” and variations of the words, for example“comprising” and “comprises”, mean “including but not limited to”, andis not intended to (and does not) exclude other moieties, additives,components, integers or steps.

Throughout the description and claims of this specification, thesingular encompasses the plural unless the context otherwise requires.In particular, where the indefinite article is used, the specificationis to be understood as contemplating plurality as well as singularity,unless the context requires otherwise.

Features, integers, characteristics, compounds, chemical moieties orgroups described in conjunction with a particular aspect, exemplaryembodiment or example of the invention are to be understood to beapplicable to any other aspect, embodiment or example described hereinunless incompatible therewith.

It will be also be appreciated that, throughout the description andclaims of this specification, language in the general form of “X for Y”(where Y is some action, activity or step and X is some means forcarrying out that action, activity or step) encompasses means X adaptedor arranged specifically, but not exclusively, to do Y.

While the invention has been shown and described with reference tocertain exemplary embodiments thereof, it will be understood by thoseskilled in the art that various changes in form and details may be madetherein without departing from the spirit and scope of the invention asdefined by the appended claims and their equivalents.

1. A method of operating a communication system comprising a NetWork(NW) and a User Equipment (UE), a Non Access Stratum (NAS) via which theNW and the UE communicate with each other, and an Access Stratum (AS)providing a physical connection to enable the NAS communication betweenthe NW and the UE to be achieved, the physical connection including awireless radio link to the UE, the method comprising: sendinginformation from the NW to the UE to determine at least one action ofthe UE in response to a failure of a radio link to the UE; detecting aRadio Link Failure (RLF); in response to the detecting of the RLF,deciding on an action to be performed by the UE according to theinformation; and performing, by the UE, the decided action, wherein thedecided action comprises at least one of triggering of a servicerequest, triggering of a Tracking Area Update (TAU), and triggering of aRouting Area Update (RAU).
 2. The method of claim 1, wherein thedeciding on the action comprises deciding whether to initiate at leastone of a service request, a TAU, and a RAU.
 3. The method of claim 1,further comprising storing the information in the UE.
 4. The method ofclaim 1, further comprising configuring the UE according to theinformation.
 5. The method of claim 4, wherein the deciding on theaction comprises deciding on an action according to the configuration ofthe UE.
 6. The method of claim 1, wherein the sending of informationcomprises sending at least one of an ATTACH_ACCEPT message, aTRACKING_AREA_UPDATE_ACCEPT message, a ROUTING_AREA_UPDATE_ACCEPTmessage, a Globally Unique Temporary Identity(GUTI)_REALLOCATION_COMMAND message, a message from an eNB to the UE,and a Radio Resource Control (RRC) signalling message.
 7. The method ofclaim 1, wherein the sending of information comprises sending at leastone of a Short Messaging Service (SMS) message, and an Open MobileAlliance Device Management (OMA DM) message.
 8. A method of operating acommunication system comprising a NetWork (NW) and a User Equipment(UE), a Non Access Stratum (NAS) via which the NW and the UE communicatewith each other, and an Access Stratum (AS) providing a physicalconnection to enable the NAS communication between the NW and the UE tobe achieved, the physical connection including a wireless radio link tothe UE, the method comprising: detecting a Radio Link Failure (RLF) bythe AS; informing the NAS of the RLF; determining if a message isreceived by the UE regarding recovery by the NAS from the RLF; if it isdetermined that the message regarding recovery by the NAS from the RLFis received, determining if the received message indicates that recoveryby the NAS from the RLF is permitted; and if it is determined that therecovery by the NAS from the RLF is permitted, performing a recoveryfrom the RLF by the NAS.
 9. The method of claim 8, wherein, if it isdetermined that the message regarding recovery by the NAS from the RLFis not received: determining if UpLink (UL) data is to be sent by theUE; and if it is determined that the UL data is to be sent by the UE,recovering from the RLF.
 10. The method of claim 8, wherein, if it isdetermined that the recovery by the NAS from the RLF is not permitted:determining if UpLink (UL) data is to be sent by the UE; and if it isdetermined that the UL data is to be sent by the UE, recovering from theRLF.
 11. The method of claim 8, wherein the determining if a message isreceived by the UE comprises: determining if at least one of anATTACH_ACCEPT message, a TRACKING_AREA_UPDATE (TAU)_ACCEPT message, aROUTING_AREA_UPDATE (RAU)_ACCEPT message, a Globally Unique TemporaryIdentity (GUTI)_REALLOCATION_COMMAND message, a message from an eNB tothe UE, and a Radio Resource Control (RRC) signalling message isreceived; and determining if the received at least one of theATTACH_ACCEPT message, the TAU_ACCEPT message, the RAU_ACCEPT message,the GUTI_REALLOCATION_COMMAND message, the message from the eNB to theUE, and the Radio Resource Control (RRC) signalling message includesinformation regarding recovery from the RLF by the NAS.